Remarkable optical properties, such as quantum light emission and large optical nonlinearity, have been observed in peculiar local sites of transition metal dichalcogenide monolayers, and the ability to tune such properties is of great importance for their optoelectronic applications. For that purpose, it is crucial to elucidate and tune their local optical properties simultaneously. Here, we develop an electric field-assisted near-field technique. Using this technique we can clarify and tune the local optical properties simultaneously with a spatial resolution of approximately 100 nm due to the electric field from the cantilever. The photoluminescence at local sites in molybdenum-disulfide (MoS2) monolayers is reversibly modulated, and the inhomogeneity of the charge neutral points and quantum yields is suggested. We successfully etch MoS2 crystals and fabricate nanoribbons using near-field techniques in combination with an electric field. This study creates a way to tune the local optical properties and to freely design the structural shapes of atomic monolayers using near-field optics.

We obtained local optical absorption spectra of MoS2 with a spatial resolution of approximately 200 nm using scanning near-field optical microscopy with a supercontinuum laser light source, and we found that the absorption spectra exhibited a significant site dependence on the MoS2 monolayer crystal. We found clear relationships between local optical absorption spectra and photoluminescence intensities. At a site that exhibited a weak photoluminescence, the intensity of the A exciton optical absorption was also weak, and its line shape was significantly deformed, which suggests the influence of n-doping. The results indicate a significant inhomogeneity in the n-doping levels on a single sheet.

Van der Waals (vdW) heterostructures, in which different two-dimensional layered materials are stacked, can exhibit unprecedented optical properties. Development of a technique to clarify local optical properties of vdW heterostructures is of great importance for the correct understanding of their backgrounds. Here, we examined local optical absorption spectra of h-BN–MoS2 vdW heterostructures by scanning near-field microscopy measurements with a spatial resolution of 100 nm. In an as-grown sample, there was almost no site dependence of their optical absorption spectra. However, in a degraded sample where defects and deformations were artificially induced, a significant site-dependence of optical absorption spectra was observed.

Understanding of the optical properties at a specific site in two-dimensional materials is important for their opto-electronic applications. Here, we investigated the site dependence of the relationship between the photoluminescence spectrum and an electric field in monolayer and bilayer molybdenum disulfide (MoS2) using a technique combining scanning near-field optical microscopy (SNOM) and a bias voltage application from the SNOM cantilever. We revealed the difference between two factors, doping level and PL quantum efficiency, at each site and their significant influence on the relationship between PL intensity and bias voltage, evaluating the specific influences of the two factors on the relationship. In addition, we clarified that carrier injection at a site in the bilayer region induces an increase in the A peak intensity and a decrease in the I peak, discussing the background in view of theoretical calculations.